1# Bionic Kernel Header Files
2
3Bionic comes with a processed set of all of the uapi Linux kernel headers that
4can safely be included by userland applications and libraries.
5
6These clean headers are automatically generated by several scripts located
7in the `tools/` directory. The tools process the original
8unmodified kernel headers in order to get rid of many annoying
9declarations and constructs that usually result in compilation failure.
10
11The 'clean headers' only contain type and macro definitions, with the
12exception of a couple static inline functions used for performance
13reason (e.g. optimized CPU-specific byte-swapping routines).
14
15They can be included from C++, or when compiling code in strict ANSI mode.
16They can be also included before or after any Bionic C library header.
17
18Description of the directories involved in generating the parsed kernel headers:
19
20  * `external/kernel-headers/original/`
21    Contains the uapi kernel headers found in the android kernel. Note this
22    also includes the header files that are generated by building the kernel
23    sources.
24
25  * `bionic/libc/kernel/uapi/`
26    Contains the cleaned kernel headers and mirrors the directory structure
27    in `external/kernel-headers/original/uapi/`.
28
29  * `bionic/libc/kernel/tools/`
30    Contains various Python and shell scripts used to get and re-generate
31    the headers.
32
33The tools to get/parse the headers:
34
35  * `tools/generate_uapi_headers.sh`
36    Checks out the android kernel and generates all uapi header files.
37    copies all the changed files into external/kernel-headers.
38
39  * `tools/clean_header.py`
40    Prints the clean version of a given kernel header. With the -u option,
41    this will also update the corresponding clean header file if its
42    content has changed. You can also process more than one file with -u.
43
44  * `tools/update_all.py`
45    Automatically update all clean headers from the content of
46    `external/kernel-headers/original/`.
47
48## How To Update The Headers
49
50IMPORTANT IMPORTANT:
51
52WHEN UPDATING THE HEADERS, ALWAYS CHECK THAT THE NEW CLEAN HEADERS DO
53NOT BREAK THE KERNEL <-> USER ABI, FOR EXAMPLE BY CHANGING THE SIZE
54OF A GIVEN TYPE. THIS TASK CANNOT BE EASILY AUTOMATED AT THE MOMENT.
55
56Download the Android mainline kernel source code:
57```
58  > mkdir kernel_src
59  > cd kernel_src
60  kernel_src> git clone https://android.googlesource.com/kernel/common/ -b android-mainline
61```
62
63For now, there are no tags, take the top of tree version. To find the
64version of the linux stable kernel headers the mainline source code is
65tracking, read the uapi/linux/version.h that is generated.
66```
67  kernel_src> cd linux-stable
68  kernel_src/linux-stable> git checkout tags/vXXX
69```
70
71Before running the command to import the headers, make sure that you have
72done a lunch TARGET. The script uses a variable set by the lunch command
73to determine which directory to use as the destination directory.
74
75After running lunch, run this command to import the headers into the android
76source tree if there is a kernel source tree already checked out:
77```
78  bionic/libc/kernel/tools/generate_uapi_headers.sh --use-kernel-dir kernel_src
79```
80
81Run this command to automatically download the latest version of the headers
82and import them if there is no checked out kernel source tree:
83```
84  bionic/libc/kernel/tools/generate_uapi_headers.sh --download-kernel
85```
86
87Next, run this command to copy the parsed files to bionic/libc/kernel/uapi:
88```
89  bionic/libc/kernel/tools/update_all.py
90```
91
92After this, you will need to build/test the tree to make sure that these
93changes do not introduce any errors.
94